1. Field of the Invention
An embodiment of the present invention relates to a glass for a light guide fiber, and more particularly relates to a glass for a light guide fiber that does not include lead.
2. Description of the Related Art
A light guide that transmits light has a configuration in which a large number of light guide fibers (hereunder, also referred to as “fibers”) are bundled together. As shown in FIG. 1, an individual fiber 10 includes a core 11 that transmits light and a cladding 12 that is provided on an outer circumferential portion of the core 11 and reflects light so that the light does not leak to outside from a side surface of the core. Glass with a high refractive index is used for the core 11, and glass with a lower refractive index than the core 11 is used for the cladding 12.
Although lead glass is known as glass with a high refractive index, glass that does not use lead (hereunder, also referred to as “lead-free glass”) has been developed for environmental reasons. For example, aluminosilicate glass that does not include lead is disclosed in Japanese Patent Application Laid-Open Publication No. 2004-256389 and Japanese Patent Application Laid-Open Publication No. 2004-277281. Further, lead-free glass that includes a rare-earth oxide and has a radiation shielding capacity is disclosed in Japanese Patent Application Laid-Open Publication No. 2009-7194. In addition, lead-free glass for a light guide is disclosed in Japanese Patent Application Laid-Open Publication No. 2009-196878, Japanese Patent Application Laid-Open Publication No. 2009-179535, and Japanese Patent Application Laid-Open Publication No. 2011-116621.
Medical endoscopes are used to perform not only normal-light imaging that uses white color light (for example, light of a wavelength between 380 and 750 nm), but also various kinds of special-light imaging that utilize wavelength characteristics of the irradiating light. For example, narrow band imaging (NBI) is a method that irradiates light of two wavelengths (390 to 445 nm and 530 to 550 nm) that have been converted to narrow-band light that is easily absorbed by hemoglobin in blood, and with which tumor tissue is easily distinguished by highlighting capillary vessels and fine mucosal patterns on the mucosal surface layers.
Further, in auto-fluorescence imaging (AFI), in order to image auto-fluorescence from a fluorescent substance present in living tissue such as collagen, narrow-band light (excitation light) of wavelengths of 390 to 445 nm and 530 to 550 nm is irradiated onto the tissue. Auto-fluorescence imaging utilizes a characteristic that, in comparison to normal tissue, tumor tissue attenuates auto-fluorescence generated by excitation light. Consequently, a high transmittance is required for glass for a light guide fiber, and in particular, a high transmittance with respect to blue color light (for example, light of a wavelength between 380 and 470 nm) is required for glass for endoscopes.
In addition, in some cases medical endoscopes are used while irradiating X-rays in order to check the position of a distal end portion or the like of the endoscope after inserting the endoscope into the body of a subject. When glass is exposed to X-rays, part of the chemical bonding thereof is broken or strained and as a result the glass is colored. Breakage and strain of chemical bonding of glass caused by radiation exposure is gradually eliminated over time by impartation of thermal energy or by energy of light that passes through the inside of the glass, and the glass also recovers from such coloring.
X-ray resistance, that is, the degree to which it is difficult for glass to be colored by X-ray exposure and the ease with which glass recovers from such coloring, changes depending on the glass composition. High X-ray resistance is required for glass for light guide fibers of a medical endoscope.